U.S. Defense Secretary Chuck Hagel says the U.S.-led airstrikes against Islamic State militants will intensify in the future as Iraqi ground forces work to become more effective.

Hagel told a U.S. congressional committee Thursday the "tempo and intensity" of the coalition's air campaign against IS will accelerate in tandem with Iraqi forces as they become stronger.

Secretary Hagel also said the United States and coalition forces are making progress in the fight against IS militants in Iraq and Syria, but that the American people must prepare for a long and difficult struggle.

He said Iraqi, Kurdish and tribal forces supported by U.S.-led airstrikes have stalled and in some cases reversed the group's advance. But he added the Islamic State is still a serious threat to American interests, U.S. allies and the Middle East, and controls large regions in western and northern Iraq and eastern Syria.

Hagel and U.S. General Martin Dempsey, chairman of the Joint Chiefs of Staff, are testifying Thursday before the House Armed Services Committee.

General Dempsey said it is possible that sometime in the future U.S. officials will consider assigning a modest number of American troops to fight with Iraqi forces in the northern city of Mosul and elsewhere.

Islamic State militants seized control of most of Mosul in June.

Their testimony comes just days after U.S. President Barack Obama authorized the U.S. military to deploy up to 1,500 more non-combat troops to Iraq as part of the mission to fight Islamic State militants.

Obama administration officials say the president will also ask Congress for $5.6 billion to help fund the campaign in Iraq. The officials say the request includes $1.6 billion to establish an Iraqi train-and-equip fund to support efforts aimed at strengthening the country’s army.